sstable readers: create a generator class for coroutines
sstable readers: create a generator class for coroutines

The data_consume_rows_context and data_consume_rows_context_m are
classes, that use primitive_consumer read* methods to get primitives
from a streamed sstable, and using their corresponding consumers' (
mp_row_consumer_k_l and mp_row_consumer_m) consume* methods, they
fill the buffer of the corresponding flat_mutation_reader.

The main procedure where we decide which read* and consume* methods
to call, is do_process_state. We save the current state of the
procedure in the _state variable, to remember where to continue in
the next call. For each call, the do_process_state method returns
an information about whether we can keep filling the buffer using
more buffers from the stream (proceed::yes), or not (proceed::no).

The saved state can be (mostly) removed by using a generator
coroutine, whose state is saved when its execution is halted,
and which yields the values, that do_process_state would return
before.

The processing_result_generator is a class for managing a generator
coroutine. When the coroutine halts, the proceed_generator saves the
value yielded by the coroutine, and returns it to the caller.

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